Multi-antenna techniques can significantly increase data rates and reliability of a wireless communication system. The performance can be in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The Long Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is support of MIMO antenna deployments and MIMO related techniques. Currently one-dimensional (horizontal) antenna array can provide flexible beam adaption in the azimuth domain only through the horizontal precoding process, while fixed down-tilt is applied in the vertical direction.
With the development of the Active Antenna Array (ASS), a full dimensional MIMO (FD-MIMO) was proposed since it was found that a full MIMO capability can be exploited through leveraging two-dimensional (2D) antenna planar such that the user-specific elevation beamforming and spatial multiplexing on the vertical domain are possible.
In LTE Rel. 13, for FD-MIMO, Channel State Information Reference Signals (CSI-RS) ports for 2D antenna arrays were increased to 12/16 for Class A CSI-RS (i.e., non-precoded CSI-RS), and antenna configurations (N1, N2)=(2,3), (3,2), (2,4), or (4,2) were used and both Code Division Multiplexing (CDM)-2 and CDM-4 were adopted for Class A CSI-RS. Moreover, technologies for further FD-MIMO enhancement are also required to be studied. For non-precoded CSI-RS enhancement, it was already proposed to extend the existing numbers {1, 2, 4, 8, 12, 16} of CSI-RS antenna ports for support of {20, 24, 28, 32} CSI-RS ports with mechanism for reducing the overhead for CSI-RS transmission.
In 36.211 d00 of Rel. 13 Agreements in RAN1 #84, there are described patterns for 12-port and 16-port CSI-RS. Regarding 16-port CSI-RS Resource Element (RE) mapping for CDM-4, an antenna configuration (N, K)=(8, 2) is used; 8 REs for mapping a legacy 8-port CSI-RS are partitioned into two groups of 4 REs; and each group of 4 REs forms a CDM group and antenna port numbering and Orthogonal Cover Code (OCC) allocation are defined in Table 1
TABLE 1Antenna Port Numbering and OCC Allocations(16-port CSI-RS RE mapping for CDM-4)CSI-RS portsOCC indicesRE locations15, 16, 19, 200, 1, 2, 3RE locations for antenna ports {15, 16,19, 20} for the first 8-port resource17, 18, 21, 220, 1, 2, 3RE locations for antenna ports {17, 18,21, 22} for the first 8-port resource23, 24, 27, 280, 1, 2, 3RE locations for antenna ports {15, 16,19, 20} for the second 8-port resource25, 26, 29, 300, 1, 2, 3RE locations for antenna ports {17, 18,21, 22} for the second 8-port resource
FIG. 1 illustrates the CSI-RS pattern for 16-port CSI-RS RE mapping for CDM-4, wherein a CDM group comprises 4 RE and two CDM group forms 8 REs for mapping a legacy 8-port CSI-RS as illustrated by bold line blocks in FIG. 1.
Regarding 12-port CSI-RS-RE for CDM-4, an antenna configuration (N,K)=(4,3) is used; 4 REs for mapping legacy 4-port CSI-RS comprise a CDM group and antenna port numbering and OCC allocation is defined in Table 2.
TABLE 2Antenna Port Numbering and OCC Allocations(12-port CSI-RS RE mapping for CDM-4)CSI-RS portsOCC indicesRE locations15, 16, 17, 180, 1, 2, 34 REs corresponding to the 1st resource19, 20, 21, 220, 1, 2, 34 REs corresponding to the 2nd resource23, 24, 25, 260, 1, 2, 34 REs corresponding to the 3rd resource27, 28, 29, 300, 1, 2, 34 REs corresponding to the 4th resource
FIG. 2 illustrates the CSI-RS pattern for 12-port CSI-RS RE mapping for CDM-4, wherein a CDM group comprises 4 REs but these 4 REs are two separate groups of REs as illustrated by bold line blocks in FIG. 2. In addition, FIG. 3 illustrate the sequence Wp′(i) for CDM-4, which is an OCC sequence with a length of 4.
For CDM-2, in 36.211 d00 of the Rel. 13 Agreements, Class A CSI-RS resource port numbers used for the k-th component resources with Rel-12 port number p′ are given as follows:
      p          (      k      )        =      {                                                                      p                ′                            +                                                N                  2                                ⁢                k                                      ,                                                                              p                ′                            =              15                        ,            16            ,            …            ⁢                                                  ,                          14              +                              N                2                                                                                                                    p                ′                            +                                                N                  2                                ⁢                                  (                                      k                    +                    K                    -                    1                                    )                                                      ,                                                                              p                ′                            =                              15                +                                  N                  2                                                      ,            …            ⁢                                                  ,                          14              +              N                                          wherein k indicates the index of the component resources, p′ indicates the Rel-12 port number, N indicates the number of ports in each component resource and K indicates the number of component resources. FIG. 4 illustrates CSI-RS pattern for 16-port CSI-RS RE mapping for CDM-2, which comprises two 8-port configurations.
In addition, it was further proposed to using CSI-RS of 0.5 RE/RB/port in RAN1#82. In the proposed scheme, not all the CSI-RS ports are located within a signal PRB but ports are multiplexed in adjacent PRB in FDM manner. In the case of normal CP 16, the CSI-RS ports include frame structure types 1 and 2 each comprising 8 CSI-RS ports. The first 8 CSI-RS ports (i.e. ports 15-22) are located in the even-numbered PRBs and the second 8 CSI-RS ports (i.e. ports 23-30) are located in odd-numbered PRBs.